Anonymous iframe: document.credentials => window.credentials?

[ArthurSonzogni]

See this discussion:
https://chromium-review.googlesource.com/c/chromium/src/+/3161670

It is possible to create document that aren't the initial empty document nor the ones created by the navigation.
For instance:

• document.implementation.createDocument(...);
• document.implementation.createHTMLDocument(...);
• document.importNode(...)
• document patched by Devtools.
• etc...

In the implementation, we are wondering if this property shouldn't be instead be part of the window object instead, because the right LocalDomWindow is associated with all of those already.
+CC @antosart